Cake: Melt chocolate and set aside. Grease and flour 2 round 9" cake pans. Preheat oven to 350. In large bowl, combine flour, sugar, brown sugar, baking soda, salt, buttermilk, shortening, eggs and vanilla. Beat with electric mixer at low speed for 1/2 minute. Increase speed to medium-high; beat 3 minutes. Pour 1 1/2 cup of mixture into a small bowl; beat in peanut butter. Set peanut butter mixture aside. Add melted chocolate to remaining mixture in large bowl. Pour 1/2 of chocolate batter into each prepared pan. With a large spoon, drop peanut-butter mixture in mounds on top of chocolate batter. Pull a narrow spatula through batter om [ams om zig-zag motion to create a marbled effect. Bake in preheated oven 30-35 minutes. Cool in pans on a wire rack. Turn out on rack, remove pans and cool completely. Frosting: Melt chocolate. In a medium bowl, combine powdered sugar, butter, vanilla, milk, and melted chocolate. Beat until smooth. Blend in peanuts or top with the peanuts.